The present invention relates to a square pipe, a basket and a spent fuel container which stores spent fuel aggregates.
A nuclear fuel aggregate, which has been burnt and is no longer used in its terminal point of a nuclear fuel cycle, is referred to as a spent fuel aggregate. The spent fuel aggregate, which contains high radioactive substances such as FP, needs to be thermally cooled off so that it is cooled off for a predetermined period (for three to six months) in a cooling pit in a nuclear power plant. Thereafter, this is housed in a cask that is a shielding container, and transported by a truck or a ship to a recycling facility where it is stored. Upon housing the spent fuel aggregates in the cask, a holding frame having a lattice shape in its cross-section, called a basket, is used. The spent fuel aggregates are inserted into cells that are a plurality of housing spaces formed in the basket, one by one, thus, it is possible to ensure an appropriate holding strength against vibration, etc. during the transportation.
With respect to conventional examples of such a cask, various types thereof have been proposed in xe2x80x9cAtomic eyexe2x80x9d (issued on Apr. 1, 1998, Nikkan Kogyo Publishing Production) and Japanese Patent Application Laid-Open No. 62-242725. The following description will discuss a cask that forms a premise upon developing the present invention. However, the cask is shown for convenience of explanation, and is not necessarily related to the conventionally known and used device.
FIG. 23 is a perspective view that shows one example of a cask. FIG. 24 is a cross-sectional view in the radial direction of the cask shown in FIG. 23. A cask 500 is constituted by a cylinder-shaped trunk main body 501, a resin portion 502 placed on the outer circumference of the trunk main body 501, an outer cylinder 503, a bottom 504 and a lid section 505. The trunk main body 501 and the bottom 504 are forged products of carbon steel that is a xcex3 ray-shielding substance. Moreover, the lid section 505 is constituted by a primary lid 506 and a secondary lid 507 made of stainless steel. The trunk main body 501 and the bottom 504 are joined to each other through butt welding.
The primary lid 506 and the secondary lid 507 are secured to the trunk main body 501 with stainless bolts. A hollow O-ring made of metal to which an aluminum coating, etc. is applied is interpolated between the lid section 505 and the trunk main body 501 so as to maintain the inside thereof in an air-tight state. Trunnions 513 which suspends the cask 500 are placed on both of the sides of a cask main body 512 (one of them is not shown). Moreover, buffer members 514 in which timber, etc. is sealed as a buffer member are attached to both of the ends of the cask main body 512 (one of them is not shown).
A plurality of inner fins 508 which allows heat conduction are placed between the trunk main body 501 and the outer cylinder 503. The inner fins 508 are made of copper as their material in order to increase the efficiency of heat conductivity. Resin 502 is injected into a space formed by the inner fins 508 in a fluid state, and solidified and formed through a thermo-curing reaction, etc. A basket 509 has a construction formed by collecting 69 square pipes 510 into a bundle as shown in FIG. 23, and is inserted into a cavity 511 of the trunk main body 501 in a fixed state. The square pipe 510 is made of an aluminum alloy in which neutron-absorbingmember (boron, B) is mixed so as to prevent the inserted spent fuel aggregate from reaching the criticality. Moreover, each housing space formed by each square pipe 510 is referred to as a cell 515, and each cell 515 can house one spent fuel aggregate. Trunnions 513 which suspends the cask 500 are placed on both of the sides of the cask main body 512 (one of them is not shown). Moreover, buffer members 514 in which timber, etc. is sealed as a buffer member are attached to both of the ends of the cask main body 512 (one of them is not shown).
A basket that has been used for a conventional radioactive substance storing container such as a cask and a canister is constituted by combining side faces of a plurality of square pipes with each other, therefore, in order to ensure a sufficient strength at the time of falling down, it is necessary to increase the plate thickness of the square pipe. For example, when a cask horizontally falls down, the load of the spent fuel aggregate is concentrated on the face end portions of each square pipe, thus, it is necessary to provide a thickness that can withstand this impact force. Moreover, since the basket needs to have a function to prevent the inserted spent fuel aggregate from reaching the criticality, the square pipe used for the basket is made of an aluminum alloy in which boron (B) is mixed as a neutron absorbing material. In order to provide this criticality preventive function, the square pipe for the basket needs to have a certain degree of thickness. For this reason, the outer shape dimension of the entire basket tends to become large, resulting in a greater mass in the entire basket.
Moreover, in order to protect the cask main body from an accident such as falling down during transportation of the cask, the buffer members 514 (one of which is omitted from the Figure) are attached to both of the ends of the cask (see FIG. 23). The impact at the time of falling of the cask is buffered with the buffer members 514 being crushed. In this case, the margin of crushing in the radial direction, which is used for buffering the impact at the time of horizontal falling down, is ensured by increasing the diameter of the buffer members 514, however, when land transportation is taken into consideration, the diameter of the buffer member 514 can not be increased unduly. When the outer diameter of the cask main body is reduced, the resulting space can be used as the margin of crushing in the buffer member 514, thereby making it possible to reduce the outer diameter of the buffer member 514.
It is an object of this invention to achieve at least one of the following points, to provide a basket which is constituted by pipes having a thickness thinner than conventional pipes and consequently to reduce the diameter of the spent nuclear fuel storing container, to provide a basket and square pipes used for storing spent fuel that are easily assembled with reduced off sets of the square pipes constituting the basket, and to provide square pipes used for storing spent fuel and a basket which can alleviate a stress concentration on a specific portion of the square pipe so as to reduce degradation in performances.
The spent fuel housing square pipe according to one aspect of the present invention comprises a plurality of square pipes assembled in a staggered arrangement. A spent fuel aggregate is housed inside the square pipes and in a space defined by walls of the square pipes. The corners of walls of each square pipe is formed into a terrace shape having a plurality of steps. When assembling the square pipes the steps of the terrace shape of adjacent square pipes are butted against each other.
These square pipes constitute a basket with a plurality of them being combined with each other in a staggered arrangement, therefore, in comparison with a basket constituted by allowing the side faces of square pipes to contact each other, it is possible to make the thickness of the side face of the square pipe thinner. This is because, if the plate thickness of the square pipe side face is equal to the size corresponding to two sheets of the conventional plate, the rigidity would virtually double the conventional rigidity. Therefore, if the plate thickness is equal to the size corresponding to two sheets of the conventional plate, it becomes possible to withstand a greater impact accordingly. Moreover, the corner portion is formed into a terrace shape, and the pipes are combined with each other by getting the step faces butted against each other, therefore, it is possible to prevent offsets in a direction perpendicular to the axis direction.
Also, the spent fuel housing square pipe has its corner portion formed into a terrace shape, therefore, strictly speaking, this is not defined as a square pipe. However, since the cross-sectional shape of this pipe perpendicular to the pipe axial direction has a square shape, and the outer shape has virtually a square shape as a whole, in the present invention, this pipe is included in the concept of square pipes. Moreover, the expression xe2x80x9csquare pipes are combined with each other in the staggered arrangementxe2x80x9d means that xe2x80x9csquare pipes are diagonally combined with each otherxe2x80x9d, and, for example, this arrangement is shown in FIG. 1. The same is also true of the following description.
The square pipes according to the present invention are used not only as a basket in which they are combined in the staggered arrangement so as to be inserted in a radioactive substance storing container such as a cask and a canister but also as a lack in a spent fuel storing pool which stores spent fuel for a predetermined period of time. In this case, the square pipes according to the present invention as they are can be used as a lack, however, it is more preferable to combine the square pipes of the present invention and use in the form of a basket. With this arrangement, after having been stored for a predetermined period of time, a plurality of spent fuel aggregates, stored in the basket, as they are, are replaced into a cask or a canister, and transported and stored so that it is possible to eliminate time-consuming tasks to replace the spent fuel aggregates into a cask, and so on one by one. The same is also true in the other aspects of the present invention.
The spent fuel housing square pipe according to another aspect of the present invention comprises a plurality of square pipes assembled in a staggered arrangement. A spent fuel aggregate is housed inside the square pipes and in a space defined by walls of the square pipes. A connecting section which assembles with a connecting section of a square pipe diagonally adjacent thereto, is formed on each of the four corners of the square pipe, and the connecting sections of diagonally adjacent square pipes is are engaged with each other.
These spent fuel housing square pipes are connected to each other in a manner so as to be engaged with a connecting section of a square pipe diagonally adjacent thereto, therefore, even when the square pipes are combined with each other, these are less susceptible to disengagement, and the basket can be easily combined. Moreover, if there should be an accidental fall, it is possible to maintain the shape of the basket more firmly. With respect to the engaging construction, in addition to the construction shown in FIG. 11, a construction using dovetail grooves and dovetail joints may be adopted.
The spent fuel housing square pipe according to still another aspect of the present invention comprises a plurality of square pipes assembled in a staggered arrangement. A spent fuel aggregate is housed inside the square pipes and in a space defined by walls of the square pipes. Corners of walls of each square pipe is formed into a terrace shape having a plurality of steps and when assembling the square pipes the steps of the terrace shape of adjacent square pipes are butted against each other. A flux trap structure, which fits to the shape of the terrace portion, is formed inside of the square pipe is at least the wall or the terrace portion of the square pipe.
In this spent fuel housing square pipe, the flux trap placed in the inside of the square pipe is allowed to have a cross-sectional shape perpendicular to the axis direction that is formed to fit to the cross-sectional shape of the corner portion having a terrace shape perpendicular to the axis direction. Consequently, the flux trap placed inside of the side face can be widened to the vicinity of the corner portion. Moreover, the flux trap placed inside of the side face makes the thickness to the outer wall virtually equal, thereby making it possible to alleviate the influence of stress concentration. At least one of the cross-sectional shapes perpendicular to the axis direction of the flux traps formed on the side face and the terrace portion of the square pipe may be formed to fit to the cross-sectional shape perpendicular to the axis direction of the corner portion having a terrace shape.
The basket according to still another aspect of the present invention comprises a square pipe assembly having plurality of square pipes assembled in a staggered arrangement, a spent fuel housing container, such as a cask or a canister, or a spent fuel storing pool, that houses the square pipe assembly. A spent fuel aggregate is housed inside the square pipes and in a space defined by walls of the square pipes.
Since this basket is constituted by combining a plurality of square pipes in a staggered arrangement, it is possible to make the thickness of the side face of the square pipe thinner than the basket that is constituted by making the side faces of the square pipes contact with each other. This is because, if the plate thickness of the square pipe side face is equal to the size corresponding to two sheets of the conventional plate, the rigidity would virtually double the conventional rigidity. Therefore, it is possible to make the outer diameter of the basket smaller, and in the case of the same outer diameter, it is possible to increase the number of spent fuel aggregates to be housed. Moreover, when this arrangement is applied to the housing rack in a spent fuel storing pool, etc., it is possible to house the spent fuel aggregates more closely, and also to lighten the system as compared with the boron-stainless product, thus, it becomes possible to reduce the load to be imposed on the structure supporting the rack at the time of any abnormal state.
The basket according to still another aspect of the present invention comprises a square pipe assembly having the square pipes disclosed above and assembled in a staggered arrangement so that spaces inside the square pipes and spaces surrounded by the side faces of the square pipes are formed into lattice-shaped cells with used fuel aggregates being housed in the cells, a spent fuel housing container, such as a cask or a canister, or a spent fuel storing pool, that houses the square pipe assembly. A spent fuel aggregate is housed inside the square pipes and in a space defined by walls of the square pipes.
This basket is constituted by combining square pipes each having a corner portion formed into, for example, a terrace shape, therefore, in addition to the functions obtained by the above-mentioned basket, offsets in the direction perpendicular to the axis direction can be regulated. Therefore, the basket is more easily combined, and if there should be an accidental fall, it is possible to maintain the shape of the basket more firmly.
The basket according to still another aspect of the present invention comprises a square pipe assembly having a plurality of square pipes assembled in a staggered arrangement, wherein corners of walls of each square pipe is formed into a terrace shape having a plurality of steps, and when assembling the square pipes the steps of the terrace shape of adjacent square pipes are butted against each other, a spent fuel housing container, such as a cask or a canister, or a spent fuel storing pool, that houses the square pipe assembly, wherein a spent fuel aggregate is housed inside the square pipes and in a space defined by walls of the square pipes, and a square pipe receiver placed between adjacent square pipes located on the outermost circumference of the square pipe assembly.
In this basket, a square pipe receiver is placed between adjacent square pipes located on the outermost circumference of square pipes constituting the basket. This square pipe receiver allows decay heat from the fuel rod aggregates inserted into cells located on the basket outermost circumference to conduct to the cask outer portion efficiently. Moreover, the impact at the time of horizontal falling of the cask is supported by this square pipe receiver, thereby making it possible to prevent collapse of the basket at the time of falling of the cask.
The basket according to still another aspect of the present invention comprises a square pipe assembly having a plurality of square pipes assembled in a staggered arrangement, a spent fuel housing container, such as a cask or a canister, or a spent fuel storing pool, that houses the square pipe assembly, wherein a spent fuel aggregate is housed inside the square pipes and in a space defined by walls of the square pipes, and a fastener which fastens the outermost square pipes of the square pipe assembly and a spacer block formed to fit to the inner shape of the spent fuel housing container or the spent fuel storing pool. In this basket, the square pipes and the spacer block are preliminarily secured by the fastening tool, such as bolts, therefore, since no machining process such machining to mount holes is required in the cavity of the canister or cask, no time-consuming tasks are required in the assembling operation.
In the spent fuel housing container according to still another aspect of the present invention, there is provided the wherein a basket, which is formed into a lattice shape as a whole by combining a plurality of square pipes with each other in a staggered arrangement, is inserted to a spent fuel housing container main body with its outer shape being fitted to the cavity inner shape of the spent fuel container main body so that a spent fuel aggregate is housed inside each of the cells in the basket.
This spent fuel housing container is provided with a basket that is formed into a lattice shape as a whole by combining a plurality of square pipes with each other in a staggered arrangement in its cavity, therefore, in comparison with the basket formed by allowing the side faces of the square pipes to contact each other, this arrangement makes the thickness of the side face of the square pipe thinner. This is because, if the plate thickness of the square pipe side face is equal to the size corresponding to two sheets of the conventional plate, the rigidity would virtually double the conventional rigidity. Therefore, since it is possible to reduce the outer diameter of the basket, it becomes possible to reduce the outer diameter of the spent fuel housing container as compared with the conventional basket. Consequently, it is possible to make the outer diameter of a buffer member to be attached to the spent fuel housing container smaller. Moreover, in the case of the same outer diameter of the spent fuel housing container, it is possible to increase the number of spent fuel aggregates to be housed.
Other objects and features of this invention will become apparent from the following description with reference to the accompanying drawings.